The present technology generally relates to a secondary battery; and a battery pack, an electric vehicle, an electric power storage system, an electric power tool and an electronic device each including the secondary battery.
Various electronic devices such as a mobile phone and a personal digital assistant (PDA) have been widely spread, and it has been demanded to reduce the sizes of the electronic devices and to prolong the lives of the electronic devices. In response to this demand, the development of a battery, particularly a secondary battery that has a small size and a lightweight and can achieve a high energy density as a power supply, has been advanced.
The application of a secondary battery has been studied to the above-mentioned electronic devices as well as other use applications. Examples of the other use applications include a battery pack which can be installed removably in an electronic device and the like, an electric vehicle such as an electric car, an electric power storage system such as an electric power server for home use, and an electric power tool such as an electric drill.
The secondary battery is equipped with a positive electrode, a negative electrode and an electrolytic solution, wherein the electrolytic solution is generally installed in the secondary battery in such a state that a separator is impregnated with the electrolytic solution.
In addition, the electrolytic solution is sometimes installed in a secondary battery in such a state that the electrolytic solution is supported by a polymeric compound. This type of secondary battery is equipped with an electrolyte layer that is a so-called gel-like electrolyte. In a secondary battery equipped with the electrolyte layer, the leakage of the electrolytic solution can be prevented.
The constitution of the electrolyte layer can largely affect the battery properties of the secondary battery. Therefore, the constitution of the electrolyte layer has been studied extensively.
Concretely, for improving a cycle property or the like, a gel polymer layer having a plurality of three-dimensional open cells that are communicated with each other has been used. For achieving an excellent electrolytic solution retention property, a closed cell polymer foam has been used. For improving a load property or the like, the percentage of the total area of a plurality of cells formed in a carrier or the like is defined. For improving a battery capacity or the like, the average diameter of a plurality of circular regions observed by the observation of the surface of an electrolyte layer is defined.